I'm sure people will disagree with me, but the MIL is an adult eating food of a tray - I'm sure she can manage to not spill any food on the sofa, or if she did, clean up appropriately. Assuming that anal retentiveness over having a clean front room is the issue, I think SIL is being very unreasonable.

With dropdeadfread though that your brother should have laughed it off and told mum to eat where she likes. Can't believe sil would behave like this. I am spectacularly grateful when mum / mil do this kind of thing for me - she wouldn't have had to buy her own ready meal either but I guess thats another issue...

I am probably less houseproud than your sil though so interesting to see what others who are not think...

agree with nappyaddict on the when in rome thing to some extent but ffs this woman has been doing them a massive favour, she is an adult who can surely eat without making a mess and they aren't even in! If I was her I would have done it anyway!